I think you hit the nail on the head with the "required automation".
Your selection maybe governed by the control panel you invest in.
The simpliest way to license is to get DataCenter CPU licenses. License the physical CPU's and then you dont need to worry about the windows licensing for the guest machines. You will still need to buy licenses for any other microsoft applications through the SPLA
You are looking at about £45 per CPU license but these prices are going up 33.4% in the UK soon
now those AMD 16 core CPU's look very attractive
For me VMware provides the best perfromance, especially with windows machines but it is too expensive for most startups and really depends on who you are targeting.
We use XEN powered by OnApp which works well but again there are some costs involved and a fair amount of development to get a half decent whmcs module put together. The default isnt much fun.
Only issue we have seen with XEN is perfromance issues with Windows machines before hte XEN drivers are installed. There is a certain amoutn of CPU hit n DOM0 for any disk IO before these are running. Once its started though speeds are good so in the real world it doesnt make that much differenence unless you are restarting your VM a lot.